An image formation system comprising forming a toner image on a transparent film by indirect dry process electrophotography and projecting the image by means of an overhead projector (hereinafter referred to as an OHP) has been widely spread. In particular, with the recent spread of indirect dry process full color electrophotographic copying machines or printers, color image formation on a transparent film for use on OHP has been increasing. Hence, a film to be transferred for electrophotography (hereinafter sometimes referred to as a film for electrophotographic transfer) which provides a projected image with satisfactory color formation has been in demand.
A conventional film to be transferred for black-and-white indirect dry process electrophotography has improved running properties and improved toner fixing properties. However, when it is used in an indirect dry process full color electrophotographic copying machine or printer, it fails to provide a satisfactory projected color image. That is, the projected image becomes cloudy, lacking color formation properties particularly in the middle tone, since the incident light from an OHP is greatly refracted at the interface between a toner image and air.
In order to solve the above problem, it is necessary to reduce the unevenness of a toner image formed on a transparent film. To this effect, various methods have been proposed to date.
For example, JP-A-59-184361 (th term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes to spray lacquer on a toner image surface. However, a binder resin of a toner is dissolved by the solvent, which deteriorates image sharpness and causes color unevenness and background stains.
JP-A-60-52861 discloses a method of covering a toner image with a laminate film. JP-A-61-36756 and JP-A-61-36762 discloses a method comprising superimposing a transparent film on a toner image, fixing the toner by applying a heat roll via the film, and then removing the film. However, these methods involve an increased number of steps after image formation and tend to destroy the toner image when a transparent film is stripped off.
JP-A-63-80273 proposes a method of fixing a toner image with a roller at a high temperature for a toner be sufficiently melted; a method of fixing using a solvent, such as toluene; a method of fixing comprising abrading the surface of a fixed toner image; and a method of fixing comprising applying a transparent coating which does not dissolve a toner to the fixed toner image. However, the roller fixing method at a high temperature is disadvantageous in that the unevenness in the areas with a lesser amount of a toner, such as a middle tone area, cannot be reduced without causing offset on a the areas with a larger amount of a toner. Use of a non-contact heat fixing apparatus, such as an oven, not only causes waviness of the transparent film but requires a considerably long fixing time for ensuring sufficient light transmitting properties. The fixing method using a solvent is disadvantageous in that the fluidity of the toner in a middle tone area cannot be increased to a degree sufficient for reducing the unevenness without causing the toner in a high density area to be destroyed and run away. The method of abrading a toner image brings about improved light transmitting properties in areas with a relatively large amount of a toner but cannot achieve sufficient reduction of unevenness in low density areas. The method of applying a transparent coating which does not dissolve a toner tends to form a distinct boundary on the toner image. It would follow that incident light is scattered at the boundary only to provide a dark, low saturation projected image.
In order to solve these problems, the following proposals have been made later.
JP-A-3-198063 discloses an OHP sheet capable of forming a smoothed toner image thereon, which has an image-receiving layer comprising a resin whose melting point is higher than the glass transition point of a toner binder resin and whose melt viscosity is lower than that of the toner binder resin. JP-A-4-125567 proposes a method for leveling the unevenness of a toner image, which comprises providing a toner image-holding layer containing a thermoplastic resin whose softening point is lower than that of a toner binder resin.
JP-A-4-212168 proposes a method comprising forming a coating layer comprising a resin whose fluidizing temperature is lower than that of a toner binder resin to give gloss to the toner image, thereby increasing color reproducibility of the projected image. JP-A-5-88400 proposes a method for eliminating unevenness of a toner image, in which a transparent resin layer having a lower apparent melt viscosity than a toner binder resin at a fixing temperature is provided.
If a resin whose melt viscosity or softening point is lower than that of a toner binder resin is used in an image-receiving layer as described above, the transparent resin of the image-receiving layer becomes soft earlier than the toner upon being heated with a fixing roll, which leads to various troubles. That is, the transparent resin layer is liable to stick to the fixing roll to cause an offset phenomenon; the image area suffers from fine waviness to make a shell-like pattern, which causes cloudiness of a projected image; and the transparent resin layer is caught by the fixing roll and, as a result, some image areas disappear, or the OHP film itself is caught and wound around the fixing roll (hereinafter the phenomenon is referred to as offset).
JP-A-2-263642 describes a method for reducing unevenness of a toner image formed on a transparent film thereby to eliminate an offset phenomenon, which comprises providing on a transparent film a transparent resin layer having a specific solubility parameter for compatibility with a toner binder resin and a higher dynamic elastic modulus than the toner binder resin at a fixing temperature. However, only the adjustment of solubility parameter is not enough for obtaining compatibility between the toner and the transparent resin sufficient for a molten toner be embedded in the transparent resin layer. Therefore, the above proposal still fails to reduce the unevenness of a toner image, only to provide a projected image having poor color formation in the middle tone area.